Conventionally, some optical modules used for an optical transmission device or the like are provided with an optical combiner for combining a plurality of light beams each having a different wavelength. The optical combiner includes, for example, band pass filters (BPFs) each of which transmits a light beam of a predetermined wavelength and reflects the other light beam to output a combined light and reflective elements each of which reflects the combined light output from the BPF, and combines the light beams of the wavelengths using a zigzag optical path formed by the BPFs and the reflective elements.
The optical module also includes photo diodes (PDs) behind laser diodes (LDs) each of which is a light source for emitting a light beam of each wavelength. The PD receives the light beam emitted rearward from the LD and monitors an optical power of the light beam of each wavelength for light beams combined by the optical combiner. The optical module uses the optical power monitored by the PD to perform automatic power control (APC) processing for constantly maintaining an output level of the LD.
In recent years, an optical module having a driver for supplying a drive signal to LDs, provided behind the LDs has been studied. The structure in which the driver is provided behind the LDs has advantages that the length of a data line over which the drive signal is propagated from the driver to the LD becomes short and degradation of the drive signal is suppressed. However, in such an optical module, because a space behind the LDs is occupied with the driver, it is difficult to dispose the PDs behind the LDs.
Accordingly, the disposition of PDs inside the optical combiner is being studied. For example, a technology is known in which a transmitting portion for transmitting part of the light beam is provided in a reflective element inside an optical combiner and the light beam passing through the transmitting portion of the reflective element is made incident on the PD provided behind the reflective element.
Patent Literature 1: Japanese Laid-open Patent Publication No. 2008-90138
However, in the conventional technology in which the transmitting portion for transmitting part of the light beam is provided in the reflective element inside the optical combiner, because the transmitting portion has no wavelength selectivity, combined light i.e. combined light including light beams of various wavelengths is incident on the PD from the BPF. Therefore, in the conventional technology, it is difficult to monitor optical power of each wavelength by the PD.
At this point, it is considered that an optical component having wavelength selectivity is additionally provided between the transmitting portion of the reflective element and the PD. However, if the optical component having the wavelength selectivity is provided additionally, the number of components are increased, which may cause the configuration to be complicated.